Restructure step 3: your server, Invite, version gate, AppGraph

- One server per app run (loopback, no password); Invite attaches an
  all-interfaces listener to the same server and Stop sharing detaches
  it (guests told with 4016); members, media and Up next never reset.
  No solo mode: you're home on your own server unless in a friend's.
- Navigation: none / yours / friend; a join passes a version gate on
  /health first (clear "they/you need to update", update offer).
- Invite as a value (host, port, name, password flag, version, fp
  reserved); addresses filtered (loopback, link-local, 198.18/15,
  bridges) and ranked; Sharing chip, Copy invite, Stop sharing.
- Alone looks like a plain player (alone replaces solo).
- AppGraph builds and disposes systems; widgets use AppScope;
  SessionState deleted; a test caps the remaining singletons.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/shell/avatar.dart

@@ -8,7 +8,7 @@ import '../session/identity.dart';
 import '../session/delivery.dart';
 import '../session/party.dart';
 import '../session/session_coordinator.dart';
-import '../state/session_state.dart';
+import '../app/app_graph.dart';
 import 'motion.dart';
 
 /// What an avatar shows: who, their picture or initial on [color], and
@@ -144,8 +144,7 @@ AvatarInfo memberAvatarInfo(MemberView m, StateBroadcast? r, SessionCoordinator
 }
 
 /// Everyone in the server, from the latest state.
-List<AvatarInfo> partyAvatars([SessionCoordinator? coord]) {
-  final c = coord ?? SessionState.instance.coordinator;
+List<AvatarInfo> partyAvatars(SessionCoordinator c) {
   final r = c.serverState.value;
   return [for (final m in r?.members ?? const <MemberView>[]) memberAvatarInfo(m, r, c)];
 }
@@ -158,7 +157,7 @@ class PartyWatch extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) => ListenableBuilder(
-        listenable: SessionState.instance.coordinator.avatars,
+        listenable: AppScope.of(context).coordinator.avatars,
         builder: (context, _) => SignalBuilder(builder: (context) => builder(context)),
       );
 }
@@ -380,8 +379,7 @@ class Pill extends StatelessWidget {
 /// Someone in a feed line or toast: by member [id] when it has one, else
 /// by display [name]; from the latest state when still there, else their
 /// initial on a stable colour.
-AvatarInfo avatarForName(String name, [SessionCoordinator? coord, String id = '']) {
-  final c = coord ?? SessionState.instance.coordinator;
+AvatarInfo avatarForName(String name, SessionCoordinator c, [String id = '']) {
   final r = c.serverState.value;
   final members = r?.members ?? const <MemberView>[];
   // By id when the line has one (names change and repeat), else by name.